Thermal spraying involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In a plasma type of thermal spray gun, a high temperature stream of plasma gas heated by an arc is used to melt and propel powder particles. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, such as a high velocity, oxygen-fuel (HVOF) gun. Thermal spray coatings of oxide ceramics are well distinguished from other forms such as sintered or melt casted by a characteristic microstructure of flattened spray particles visible in metallographically prepared cross sections of coatings.
In one group of thermal spray materials, powders are formed of oxides for spraying coatings that are used for thermal insulation at high temperature such as on burner can surfaces in gas turbine engines. Coatings are also needed for erosion and wear protection at high temperatures, and require resistance against thermal cycle fatigue and hot corrosion in a combustion environment. Zirconium dioxide (zirconia) typically is used in such applications. Because of phase transitions, the zirconia is partially or fully stabilized with about 5% (by weight) 15% calcium oxide (calcia) or 6% to 20% yttrium oxide (yttria). However, these coatings have limitations particularly in resistance to hot corrosion as they allow attack of the substrate or a bond coating
Dicalcium silicate (Ca.sub.2 SiO.sub.4) is a ceramic conventionally used for cement and refractory applications. Excellent hot corrosion and heat resistance of dicalcium silicate based coatings also has been demonstrated in a high temperature combustion environment. However, it is polymorphic with at least five phases including three high temperature .alpha. modifications, an intermediate temperature monoclinic .beta. phase (larnite) and an ambient temperature .gamma. phase. The transformation from the .beta. phase to the .gamma. phase exhibits a volume increase of 12% leading to degradation in both the thermal spray process and the coatings in thermal cycling. The .beta. phase may be retained by quenching or the use of a stabilizer such as sodium or phosphorous. Other suggested stabilizers include oxides (or ions) of sulphur, boron, chromium, arsenic, vanadium, manganese, aluminum, iron, strontium, barium and potassium. At least some of these have also been reported as unsuccessful, and therefore still questionable in stabilizing, including chromium, aluminum, iron, strontium and barium.
U.S. Pat. No. 4,255,495 (Levine et al.) discloses plasma sprayed coatings of thermal barrier oxides containing at least one alkaline earth silicate such as calcium silicate. U.S. Pat. No. 5,082,741 (Tiara et al.) and an article "Advanced Thermal Barrier Coatings Involving Efficient Vertical Micro-Cracks" by N.Nakahira, Y.Harada, N.Mifune, T.Yogoro and H.Yamane, Proceedings of International Thermal Spray Conference, Orlando Fla., May 28-Jun. 5, 1992, disclose thermal spray coatings of dicalcium silicate combined with calcium zirconate (CaZrO.sub.3) in a range of proportions.
A commercial powder of .beta. phase dicalcium silicate for thermal spraying is sold by Montreal Carbide Co. Ltd., Boucherville CQ, Canada, indicated in their "Technical Bulletin MC-C.sub.2 S" (undated).
In a chemical analysis the present inventors measured less than 1% by weight of potential stabilizers such as phosphorous in Montreal Carbide powder.
A commercial powder of dicalcium silicate for thermal spraying also is sold by Cerac Inc., Milwaukee, Wis. In a Certificate of Analysis for calcium silicate (Oct. 20, 1997), Cerac reports major .beta. phase and low levels of aluminum (0.12%), iron (0.1%) and magnesium (0.25%), and 0.02% or less of other elements.
An object of the present invention is to provide an improved powder of dicalcium silicate for thermal sprayed coatings for thermal barriers having resistance to hot corrosion and sulfidation in a combustion environment. A further object is to provide a novel process of manufacturing such a powder. Another object is to provide an improved thermal sprayed coating of dicalcium silicate for thermal barriers having resistance to hot corrosion and sulfidation in a combustion environment.